This invention is concerned with improvements in and relating to polymers of vinylidene chloride. More specifically, it relates to a method for lowering the amount of unreacted vinylidene chloride which is contained in a vinylidene chloride polymer.
It is known that many polymerization reactions do not proceed quantitatively, that is, that a variable amount of unreacted or "free" monomer remains when polymerization stops. In the case of vinylidene chloride polymers and copolymers, although polymerization can easily be carried to greater than 99% completion, a fraction of a percent of the monomers generally remains unreacted. The reason for this is not always understood, for it sometimes happens when active polymerization initiator remains in the reaction system.
Polymers and copolymers of vinylidene chloride (referred to hereinafter collectively as polymers) are highly useful as coatings on a variety of substrates, including such things as various types of packaging films, such as regenerated cellulose, polyester and polyolefin films. Although the amount of free vinylidene chloride in the final coated article is ordinarily reduced to no detectable amount during the drying step of the coating process, it is nevertheless desirable to lower the amount of unreacted vinylidene chloride contained in such polymers as prepared, so as to lower the amount of vinylidene chloride vented from the coating tower with the exhaust drying gases, and to minimize the exposure of personnel working in or near the coating station to vinylidene chloride gas which vaporizes from the coating bath. Such vaporization occurs from both aqueous dispersions and organic solvent solutions of such polymers.
In British Pat. No. 573,369 there is disclosed a polymerization process for vinylidene chloride polymers which employs a persulfate initiator and a compound capable of taking up molecular oxygen such as potassium meta-bisulfite, for the purposes of increasing the yield of polymer and decreasing the time required for polymerization. However, such a process has been found in practice to give polymers which still contain unreacted monomers including vinylidene chloride in greater or lesser amount.
Consequently, one method widely employed for removing unreacted monomer from an aqueous polymer dispersion is by "devolatizing" or stripping, wherein unreacted monomer is removed by vaporizing it, usually under reduced pressure and generally with heat. Such is disclosed, for example, in U.S. Pat. No. 2,989,517 (Column 6, lines 63-65, and Column 8, lines 45 ff.) and in U.S. Pat. No. 3,879,359 (Column 3, lines 67-68). Even more complex stripping processes are known, such as that described in U.S. Pat. No. 2,713,568, wherein a small amount of an alkyl acrylate is added to the polymer dispersion just in advance of stripping the unreacted monomers therefrom. All such procedures, however, require substantial investment in distillation equipment including heaters, vacuum pumps, condensers, etc., and furthermore are expensive to operate on a commercial scale in view of the excessive amount of energy required to remove the small amount of monomer in question.